The present invention relates to an apparatus for recording data on and reproducing data from, a recording medium having concentric tracks or a spiral track, and more particularly to an apparatus for controlling the tracking of the recording medium.
Recently various apparatuses for optically recording data on and reproducing data from, a disk-shaped recording medium such as an optical disk have been proposed. To record or reproduce data, the disk-shaped recording medium is spinned, and a laser beam is applied to the concentric track or spiral track of the spinning recording medium. The thinner the beam, the higher the recording density of the medium. The recording density is high since the laser beam has a small diameter. The pitch, at which the concentric tracks or the turns of the spiral track are arranged, is only a few microns. Due to the changes of the manufacture conditions of the disk-shaped recording medium, e.g., changes in the press force, the tracks are not perfectly concentric or not perfectly spiral. Further, the center hole of the medium, into which the center spindle of the apparatus is inserted, may not be completely concentric, or the shaft of the motor for spinning the medium may not be coaxial with the medium. Hence, the tracking of the medium must be accurately achieved to record or reproduce data. In other words, the laser beam spot must be accurately moved in the radial direction of the medium to a target concentric track or a target turn of the spiral track.
A tracking control apparatus with a two-stage servo, closed loop control system is known. It performs the so-called tracking control (hereinafter called "lens tracking") achieved by moving the objective lens or mirror provided in an optical pickup. It performs also the so-called access control (hereinafter called "carriage tracking") effected by moving a carriage and, thus, moving the pickup mounted on the carriage. The beam reflected from an optical disk is detected by a photo-detector and converted into a tracking error signal which represents the distance between the beam spot and the target track. The tracking error signal is supplied to a lens tracking circuit and a carriage tracking circuit. The lens tracking circuit drives an actuator. The actuator move the lens in the radial direction of the disk until the tracking error signal falls to zero level. The carriage tracking circuit drives a voice coil motor or the like which in turn moves the carriage in the radial direction of the disk until the tracking error signal falls to zero level.
The tracking control apparatus has the drawback that the closed loop control system is complicated and expensive. Since the lens tracking circuit quickly responds to the high-frequency tracking error whereas the carriage tracking circuit quickly responds to the low-frequency tracking error but slowly responds to the high-frequency tracking error due to the weight of the carriage, complicated circuits must be used to compensate for this difference in frequency characteristic between the two tracking circuits.
The tracking control apparatus has the other drawback that the control signal undergoes a phase delay inevitably due to the response delay of the carriage. To avoid this delay, a mechanism which can quickly move the carriage must be used. Such a mechanism is expensive.